The present invention relates to a positive-working photoresist composition or, more particularly, to a positive-working photoresist composition suitable for use in fine patterning of a photoresist layer in the manufacturing process of semiconductor-based devices such as LSIs, VLSIs and the like.
Recent progress in the semiconductor technology day by day has aroused a rapidly increasing demand for computerized instruments including computers for industrial use, instruments for automatization of offices, personal computers and the like and, corresponding thereto, semiconductor devices such as integrated circuits are under an overwhelming trend toward increase in the density or degree of integration. For example, the times are already entering the era of VLSIs having a density of 1 megabit or higher after passing through the ages of 256 kilobits. Such a high density of integration in VLSIs naturally requires extremely fine patterning on semiconductor wafers in the so-called submicron range. For example, the minimum line width to be reproduced with high fidelity in the photoresist layer is about 2 .mu.m in semiconductor ICs for 256 kilobits DRAMs, about 1.0 to 1.3 .mu.m in devices for 1 megabit DRAMs and about 0.5 to 0.8 .mu.m is devices for 4 megabits DRAMs so that the technology of patterning must comply with such an extremely high precision.
As is known, the patterning works on the semiconductor wafers for the manufacture of integrated circuits are performed by the technology of photolithography using a photoresist composition. Of the two types of photoresist compositions including positive-working and negative-working ones, the positive-working photo-resist compositions are preferred widely in the works of fine patterning in which high-fidelity reproduction of a line pattern having a width of 1 to 2 .mu.m is essential.
The principal ingredients in most of the conventional positive-working photoresist compositions are an alkali-soluble novolac resin as the film-forming constituent and a quinone diazide compound as the photodecomposable or photosensitive constituent in the form of a mixture. Typical photosensitizer compounds of the quinone diazide type include sulfonic acid esters formed between a naphthoquinone diazide sulfonic acid and a compound having one or more of phenolic hydroxy groups disclosed in U.S. Pat. No. 3,402,044 and other esters disclosed in U.S. Pat. No. 3,046,118, No. 3,106,465 and No. 3,148,983.
In respect of the film-forming constituent of the photoresist compositions which is typically an alkali-soluble novolac resin, on the other hand, various types of novolac resins have been proposed including phenol formaldehyde novolac resins disclosed in U.S. Pat. No. 3,402,044 and cresol novolac resins disclosed in Electrochemistry and Industrial Physical Chemistry, volume 48, page 584 (1980). Further, Japanese Patent Kokai 58-17112 teaches that the sensitivity of a positive-working photoresist composition comprising a cresol novolac resin as the film-forming constituent can be improved by suitably selecting the proportion of the cresol isomers in the cresol used in the preparation of the cresol novolac resin.
Turning now to the problems in the process of exposure of the photoresist layer to light, the exposure is carried out either by the exposure by direct contacting or by the exposure by minifying projection. In the former method of contacting exposure, the photoresist layer formed on the surface of a semiconductor wafer is exposed to light through a patterned photomask in direct contact with the photoresist layer. This method is advantageous in respect of the contrast of the patterned image so that a patterned photoresist layer of a considerably high contrast can be obtained by this method even when the photoresist composition used there is inherently inferior in respect of the contrast and fidelity of the pattern reproduction. On the contrary to the advantages, this method has some disadvantages and problems. For example, the photomask is sometimes damaged mechanically as a natural consequence of direct contact with the photoresist layer in each time of exposure so that extreme care is required in handling the photomask and good quality of the photomasks can be maintained only with expenses more than negligible. Moreover, needless to say, the pattern on the photomask must be of the full-size relative to the pattern to be reproduced so that a patterned photomask having such a high precision is unavoidably very expensive, especially, when the line width of the pattern is in the submicron range.
In the method of exposure by minifying projection, on the other hand, the dimension of the pattern on a patterned photomask can be as large as 5 to 10 times of that in the photoresist pattern to be reproduced so that even a photomask of high precision for patterning in the submicron range can be obtained with a relatively low cost. On the contrary to this advantage, this method is disadvantageous in respect of the contrast of light between the areas to be exposed and not to be exposed in comparison with the exposure by direct contacting of the photomask. Therefore, this method of exposure by minifying projection is applicable to the reproduction of a pattern of high precision only when the photoresist composition is inherently highly sensitive in exposure to light with relatively low contrast.
In the manufacture of semiconductor devices such as VLSIs, furthermore, the pattern to be formed in the photoresist layer is not composed of lines having one and the same line width but includes lines having varied line widths combined in a complicated manner. This fact causes a difficult problem affecting the quality of pattern reproduction since the minimum exposure dose by which the photoresist layer on the exposed areas can be removed away by development considerably depends on the line width. Taking the minimum exposure dose to a pattern of 2.0 .mu.m line width as unity, for example, the minimum exposure does to patterns of 1.5 .mu.m and 1.0 .mu.m line widths are 1.2 to 1.3 and 1.5 to 1.7, respectively. Therefore, an exposure dose which is optimum for a line of certain line width may be too small or too large for lines having a smaller or larger line width, respectively, in the same pattern to cause insufficient reproduction of finer lines or excessive removal of the photoresist layer of the coarser lines so that the fidelity of pattern reproduction cannot be highest over whole area of the pattern. Moreover, the surface of a semiconductor device under way of processing is not completely flat but usually has a stepwise height difference of 0.5 to 1.0 .mu.m from portion to portion so that the thickness of a photoresist layer formed on such a stepwise surface cannot be uniform to be smaller in the upside area of the step and larger in the downside area of the step. When such a photoresist layer is exposed to light and developed, therefore, it is usual that the line width of the pattern reproduced in the photoresist layer is smaller in the area where the photoresist layer has a smaller thickness than in the area where the thickness is larger affecting the fidelity of pattern reproduction.
In connection with the process of etching on the surface of a semiconductor wafer on which a patterned photoresist layer of submicron fineness is formed, an undesirable phenomenon of side etching is unavoidable more or less in a wet process so that the process of etching is sometimes performed by a dry process free from side etching by use of plasma. In this dry etching method, however, the patterned photoresist layer as the etching mask is attacked by the plasma to cause gradual reduction in the film thickness. Accordingly, it is a desirable condition that the patterned line of the photoresist layer has a cross section in which the width of the line is not affected even when the film thickness is reduced by the attack of the plasma in the process of dry etching.
The above described problems each concern the poor reproducibility or fidelity between the original of the pattern on the photomask and the patterned image reproduced in the photoresist layer. The reasons therefor include, as is mentioned above, the decrease in the contrast in the exposure by minifying projection between the exposed and unexposed areas, difference in the optimum exposure doses between line patterns having different line widths, difference in the film thickness of the photoresist layer between the areas on both sides of a step on the wafer surface having stepwise height differences, and so on.
These problems can be solved as a whole only by the use of a photoresist composition having high fidelity in pattern reproduction and free from the influence of the exposure dose on the dimensions of the reproduced pattern. The photoresist composition free from the influence of the exposure dose on the dimensions of the reproduced pattern here implied should have following characteristics. Namely, the reproduced line pattern should have a line width which is an accurate reproduction of the line in the original pattern on the photomask without expansion or diminishment irrespective of the exposure dose or the extent of development. The patterned line of the photoresist layer should have a rectangular cross section standing on the substrate surface with definitely angled shoulders while undesirable cross sectional configurations include those having trailing skirts on the substrate surface even with definitely angled shoulders because the photoresist layer may disappear in the thin skirt portions by the attack of the etching plasma to cause a change in the line width of the photoresist pattern.
The specification of U.S. Pat. No. 4,587,196 recently published discloses a positive-working photoresist composition having a high sensitivity in exposure to light. The composition is formulated with a cresol novolac resin and a photosensitizing agent, of which the cresol moiety in the cresol novolac resin is derived from an isomeric mixture of o-, m- and p-cresols in a specified proportion. In particular, a preferable isomeric isomer of cresols contains at least 47% of m-cresol, the balance being p-cresol optionally combined with o-cresol. According to the disclosure, the composition is imparted with a greatly improved sensitivity to light as compared with similar compositions in which the cresol novolac resin is derived from an isomeric mixture of cresols with a smaller proportion of m-cresol. When the proportion of m-cresol is decreased to 45%, the balance being p-cresol, for example, the photosensitivity of the resultant photosensitive resin composition is greatly decreased. Despite the improvement in the sensitivity as taught there, such a photosensitive resist composition derived, for example, from a cresol mixture of 50% m-cresol and 50% p-cresol is not quite satisfactory in respect of the fidelity in the pattern reproduction in the sense mentioned above. Accordingly, it is eagerly desired to develop a photosensitive resist composition capable of giving a resist pattern of high fidelity because this is the key requirement to comply with the recent technological trend in the photosensitive resin composition assuming that a more powerful light source can be used to compensate the decrease in the photosensitivity.